Marco,
I don't quite understand your SQL. I normally use subselects when I want
the row to appear even if there's no match for the subselect, but in your
case it seems like you always want it to match and then there's no need to
make it a subselect. Simply change to

SELECT OPC.ESERCIZIO_ID,
OPC.CONDOMINIO_ID,
OPC.CONTO_ID,
OPC.DESCRIZIONEDISP,
SSPC.RIPARTITO
FROM PIANO_CONTI OPC
JOIN PIANO_CONTI SSPC ON SSPC.ESERCIZIO_ID=OPC.ESERCIZIO_ID
WHERE SSPC.RIPARTITO='S'

HTH,
Set